Intramuscular medication as the name implies is a long-acting injectable form of medication that is usually administered into the gluteal muscle or deltoid muscle and it’s designed to take the place of PO or oral formulations.

The medications available in IM formulations: 

• Aripiprazole (Abilify Maintena) 
• Aripiprazole lauroxil (Aristada) 
• Fluphenazine (prolixin)
• Haloperidol (Haldol)
• Olanzapine pamoate (Zyprexa Relprevv) 
• Paliperidone (Invega Sustenna, Invega Trinza) 
• Risperidone (Risperdal Consta) 

Most last between 2-4 weeks but medications like Invega trinza lasts up to 3 months 

This solves one of the major issues when prescribing medication, which is adherence with treatment. 

Notice that all these medications are first- or second-generation dopamine blockers. These medications are commonly used to treat disorder like Bipolar and Schizophrenia (serious mental illness). These populations often have difficulty with medication adherence. 

Clinically most psychiatrists will tell you IM medication improves patient outcomes. However, they may not outperform PO medication taken daily and consistently. Where these medication formulations have the biggest impact is for people who had improvement on oral medication but often forget to take medication or do not want to take medication daily. Many patients with serious mental illness stop taking medication when symptoms resolve making relapse likely. 

Side effects will be similar to the oral medication with the added logistical issue of coming to the office for the injection, and pain at the injection site. Normally we assess tolerability and risk of side effects with oral medication before giving IM medication. This avoids the potential for long lasting side effects.